1. Field of the Invention
The present invention relates to a method for producing a hydrogen storage alloy electrode which employs a hydrogen storage alloy which can absorb and desorb hydrogen gas in a reversible manner.
2. Description of the Related Art
Storage batteries, which are widely used as power sources in a variety of applications, are typically classified into two general groups of lead-acid storage batteries and alkaline storage batteries. Of the two groups, alkaline storage batteries are more reliable, and can be made smaller and lighter. Small alkaline storage batteries are generally favored for portable electric appliances; large alkaline storage batteries have been used mainly in conjunction with industrial equipments.
Some alkaline storage batteries use, for example, air electrode or silver oxide electrode for their positive electrode; however, more commonly nickel electrode is used for the positive electrode. Nickel electrodes have been popular particularly because they were reconfigured from a pocket type to a sintered type for improving their characteristics, and became even more popular with the development of hermetic-sealing.
Cadmium is most commonly used to form the negative electrodes of alkaline storage batteries, although other materials including zinc, iron, hydrogen and the like have also been employed.
There is a considerable commercial interest in the storage batteries that have a higher energy density than the batteries currently available. In an attempt to achieve this goal, researchers have investigated nickel-hydrogen (metal hydride) storage batteries which incorporate hydrogen storage electrodes. There have been made a number of proposals on the production method of the hydrogen storage alloy electrodes using the metal hydrides.
The alloys in these electrodes or the hydrides form of such alloys, can absorb and desorb hydrogen in a reversible manner, and thus the alloys and the electrodes made of these alloys have come to be known as hydrogen storage alloys and hydrogen storage electrodes (or hydrogen storage alloy electrodes), respectively.
Batteries made with hydrogen storage electrodes have a large theoretical energy density as compared with the batteries formed with cadmium electrodes. Also, batteries that employ hydrogen storage electrodes are free from the formation and the subsequent deformation of dendrites, which are problems involved in the zinc electrodes. These advantageous properties of the batteries employing hydrogen storage electrodes, as well as the promising longer service life and the reduction in the environmental concerns inherent to zinc and cadmium containing electrodes/batteries, have encouraged us to develop alloys suitable for the hydrogen storage electrodes, particularly negative electrodes for alkaline storage batteries.
Prior art alloys for hydrogen storage electrodes include multi-component alloys such as those of either the Ti--Ni system alloys, and the La (or Mm)--Ni system alloys (wherein Mm is a misch metal). The multi-component alloy of the Ti--Ni system is classified as an AB type (wherein, A is La, Zr, Ti or an element with a similar affinity for hydrogen, and B is Ni, Mn, Cr or any other transition metal). When this type of alloy is used as the negative electrode in an alkaline storage battery, the electrode exhibits a relatively large discharge capacity during the initial charging and discharging cycles. However, electrodes comprising these alloys do not maintain their large discharge capacity after repeated charging and discharging cycles, i.e., do not have a large saturation discharge capacity.
Another multi-component alloy is of the La (or Mm)--Ni system, which is classified as an AB.sub.5 type, wherein A and B are defined as above in relation to the AB type alloy. The alloys of this system have recently been developed in various research projects and thus been regarded as a relatively promising alloy material for the electrodes up to the present. However, the alloys of this system suffer several disadvantages such as relatively small discharge capacity, insufficient service life performances as the electrodes of the batteries, and use of costly materials. Therefore, it is eagerly desired to develop novel alloys from which hydrogen storage electrodes having a large discharge capacity and a long cycle life can be realized.
In order to overcome the above-discussed disadvantages and deficiencies, there has been proposed a method of treating the hydrogen storage alloy powder or the hydrogen storage alloy electrode by immersing it in an alkaline solution.
The conventional method of treating the hydrogen storage alloy powder or the hydrogen storage alloy electrode by immersing it in an alkaline solution is aiming at an acquisition of the following two effects: One is to improve the electrochemical activity of the alloy powder or the alloy electrode by dissolving and oxidizing metal element such as cobalt, manganese or vanadium exposed on the surface of the alloy, thereby increasing the amount of metal nickel (Ni) exposed over the surface of the alloy. The other is directed to the improvement in the battery cycle life or endurance by removing component elements on the surface of the alloy powder or the alloy electrode dissolving in the alkaline solution.
Under the circumstances wherein the nickel-metal hydride storage battery is required to have a more favorable high-rate discharge characteristic and an improved low-temperature high-rate discharge characteristic, it is however difficult to satisfy the above-mentioned requirement only by the prior art alkali treatment of the alloy powder or the alloy electrode. That is, an excessive degree of alkali treatment deviating from an optimum condition undesirably increases the amount of the oxides on the surface of the alloy powder or the alloy electrode, enhances the contact resistance among the alloy particles, and decreases the electrochemical activity of the alloy powder or alloy electrode, thereby deteriorating the battery characteristics.